The present invention relates to a vibrato signal generating arrangement for an electronic musical instrument.
An electronic musical instrument is generally accompanied with a vibrato device. In the electronic musical instrument, it is more preferably to add vibrato after the lapse of a relatively short time from key depression and then increase such vibrato gradually than to add vibrato immediately after key depression. The former vibrato is known as the delayed vibrato. In such delayed vibrato, a clear tone pitch of the depressed key may be obtained at time of key depression.
A typical example of existing delayed vibrato devices is provided with a series combination of a resistor and a capacitor connected between a DC power supply and the ground, an astable multivibrator for generating vibrato signals connected in parallel with the capacitor with one end connected the ground, a one-shot multivibrator triggered by a trigger pulse obtained at key depression to generate an output pulse with a predetermined duration, and a switch circuit connected in parallel with the capacitor and allowed to conduct by the output pulse from the one-shot multivibrator.
According to the aforesaid vibrator device, the capacitor discharges for the duration (about 100 ms) of the output pulse from the one-shot multivibrator, and thus the astable multivibrator generates no vibrator signal. After dissipation of the output pulse from the one-shot multivibrator, the capacitor charges gradually. On start of charging of the capacitor, the astable multivibrator starts to oscillate and generate an output signal with an amplitude depending on the voltage across the capacitor. Namely, the output amplitude of the astable multivibrator increases gradually.
The above-mentioned vibrato device has a somewhat complicated construction due to the provision of the one-shot multivibrator. The output amplitude of the astable multivibrator fluctuates between the supply voltage and the ground potential. Therefore, the DC level at the output of the astable multivibrator fluctuates while the output amplitude of the astable mulitivibrator varies. Thus, even though the astable multivibrator is coupled to a tone generator through a coupling capacitor, the average pitch of a tone generated will vary due to the fluctuation of the DC level.